Integrated circuits are electronic circuits which normally include a very large number of semiconductor elements, such as transistors and diodes, as well as other electronic components in highly compact form on a silicon wafer. The basic principle underlying integrated circuits is that instead of building an electronic circuit out of discrete components such as transistors, diodes, resistors and capacitors, the entire circuit is fabricated on the same piece of silicon in the form of a number of superimposed layers of conducting, insulating and transistors forming materials. By arranging predetermined geometric shapes in each of these layers, a circuit having a required function is realized.
The process by which an integrated circuit is fabricated is long and complex and basically includes the following steps:
1. A polished silicon wafer is exposed to an appropriate material in a reactor in order to "grow" a uniform layer of the material on its surface.
2. After the wafer is removed from the reactor, it is coated with a thin film of resist material, which is dried and baked.
3. A mask for the appropriate layer is created, which defines the precise pattern required on the silicon surface.
4. This mask is exposed to intense radiation such as ultraviolet light or low energy X-rays. In areas wherein the mask is transparent to such radiation, the radiation passes into the resist and breaks down its molecular structure. The portions of the resist that are covered by opaque portions of the mask are not affected.
5. The resist is then developed by immersing the silicon wafer in a solvent which dissolves the resist only at the locations which were exposed to the radiation.
6. The wafer is then exposed to an appropriate etching material which is effective to remove the material grown previously thereon (Step 1), except where the material is covered by resist, thus defining a desired pattern of the grown material.
7. The remaining resist material is then removed.
The above process is repeated for each of the layers of material forming the integrated circuit. In most integrated circuit technology one or more layers of metal are employed for internal connections on the integrated circuit. These metal layers are usually formed last.
As can be readily appreciated, the above process requires sophisticated machinery and custom masks which must be specially set up for each specific type of custom integrated circuit to be fabricated. For the process to be at all economical, extremely large quantities of integrated circuits must be produced for a given set up. Since there exist applications wherein the quantities desired of a specific circuit are not extremely large, techniques have been developed for producing integrated circuits known as gate arrays. In these gate arrays, a large amount of transistors and other components are produced by the above-described mass production techniques. Specific interconnections therebetween suitable for a specific circuit are then formed on the chip by etching a suitable metal layer thereon. Normally a protective insulative layer, such as SiO.sub.2 is formed over the metal layer for mechanical protection as well as electrical insulation.
Despite the use of sophisticated computer aided design and semi-custom devices, the widespread use of dedicated non-standard integrated circuits is still limited because of the high costs and long lead time involved in their fabrication. Many users of integrated circuits still make extensive use of standard, "off the shelf" available small scale and medium scale integrated circuits.
Electronic circuits manufactured by combining a large number of such standard integrated circuits have a number of disadvantages:
a. A large number of printed circuit boards are required, resulting in an expensive product, relatively long assembly time, relative difficulty in maintenance and troubleshooting and large size.
b. Lower electrical power efficiency is realized. As a result, relatively large power sources are required and heavier and larger products result.
There are known integrated circuits such as EMPROMS and PALS (programmable array logic) which are programmed by fusing which is accomplished electrically via the integrated circuit's input/output pins, requiring elaborate extra fusing circuitry for this purpose.
There are also known techniques for increasing yield of integrated circuits by excising inoperative portions thereof by fusing. This is normally done on wafers including a multiplicity of integrated circuits.